This invention relates to a novel process for producing polyesters. More particularly, this invention relates to a continuous, integrated, multistep process for producing high molecular weight polyester involving the direct esterification of a dicarboxylic acid with a dihydric alcohol to form a monomeric product to be susequently polymerized in a minimum of stages to polyesters characterized by uniformity and intrinsic viscosities heretofore unobtainable.
Film and fiber-forming linear polyesters are generally prepared by initially converting the free dicarboxylic acid or acid-forming derivative thereof to the dialkyl ester which is then reacted with the desired dihydric alcohol or glycol under the influence of a transesterification catalyst to form the glycol ester or monomer. This ester product is polycondensed in the presence of a suitable catalyst, splitting out excess diol, under reduced pressure and elevated temperatures to form high molecular weight polyesters suitable for conversion into molded and extruded articles. The polymerization or polycondensation phase of the overall process is usually conducted in a plurality of reaction vessels, each succeeding vessel operating under lower pressure and higher temperature than the preceeding one until desired or maximum product intrinsic viscosity is reached. In accordance with certain prior art teachings, the polycondensation reaction may be subdivided into "low," i.e. product attains an intrinsic viscosity up to about 0.1 to 0.4, preferably about 0.25 and "high" polymerization stages, each stage, and particularly the latter, requiring more than one independent reactor.
When the ester interchange process is employed, that is, the monomer is formed by the transesterification reaction hereinbefore described, certain process disadvantages are present which, in turn, limit the usefulness of the product resulting therefrom. Obviously, the first disadvantage exists at the beginning of the process since an intermediate product, the dialkyl ester, must be produced prior to formation of glycol ester monomer. Aside from the pure economic deficiencies of such a process, catalytic and alcoholic impurities are readily introduced into the system which propagate side reactions during polycondensation and limit the length of the polymeric chain obtained. The latter result is particularly detrimental in terms of final product properties since a polymeric ester of longer average chain length of repeating monomeric ester units will have a higher molecular weight corresponding to increased intrinsic viscosity. Naturally, high intrinsic viscosities are desired, especially where products such as continuous filaments suitable for tire reinforcement are contemplated, due to the higher tenacities possible in conversion articles. With employment of the transesterification process, intrinsic viscosities as high as 1.0 are rarely obtainable, the intrinsic viscosity generally residing in the range of about 0.4 to 0.9.
If a single factor were to be indicated as primarily accounting for the deficiencies associated with the transesterification process, it would be the presence during polycondensation of alkyl alcohol, particularly methanol as in the case of dimethyl terephthalate intermediate, which functions as a "chain stopper", thus effectively limiting molecular weight and intrinsic viscosity. The ester interchange reactionm as with virtually all chemical reactions under defined conditions, reaches an equilibrium point involving oppositely competing reactions as opposed to proceeding to 100 percent transesterification. Adjustment of reaction conditions to drive the reaction to as near 100 percent completion as possible, involving the employment of complicated equipment and impractical process parameters, can be attempted. It is only under extreme conditions, however, that the reaction approaches the percent ester interchange required, i.e. about 99.8 percent in the case of ethylene terephthalate monomer prepared by reacting dimethyl terephthalate and ethylene glycol, for the production of high molecular weight polymers, i.e. intrinsic viscosities of about 0.8.
Recently, it has been proposed to eliminate the above problems associated with the ester interchange process by directly esterifying dicarboxylic acid with desired dihydric alcohol, i,e, direct esterification of terephthalic acid with ethylene glycol in the case of polyethylene terephthalate, prior to polymerization. In large part, the direct esterification route became a commercial possibility through the development of terephthalic acid of relatively high purity. Depending upon contaminants present, terephthalic acid of about 90 to 100 percent purity will not significantly affect final product properties, particularly melting point and polymer color.
This path to high molecular weight polyester production incorporates obvious economic benefits into the overall process while enabling the production of a product of higher intrinsic viscosity with the resulting enhancement of converted article properties. For example, the direct esterification method does not entail the use of the transesterification catalysts, well-know color formers and insolubles removed with difficulty, during monomer formation nor does it requires a high percent ester conversion. In fact, since chain stoppers are not inherently present or generated during monomer production, a relatively low percent completion of the direct ester-forming reaction, i.e. about 50 to 75 percent where desired, still enables the theoretical preparation of polyesters of high purity and intrinsic viscosity, i.e. intrinsic viscosities within the range of about 1.0-1.5 and higher where desired. Of course, products of lower intrinsic viscosity can be produced, i.e., 0.4-0.6, but it should be realized that the benefits of the direct esterification path are more apparent with respect to higher molecular weight polyester.
However, the theoretical ability to attain a high molecular weight product is not in itself determinative of the ability to produce polyester product of fiber and film-forming quality or polyester suitable for use in other extrusion and molding applications. In order to achieve the ultimate objections of high quality and reproducibility coupled with high intrinsic viscosity, side reactions, particularly ether formation, must also be minimized. Although the polymeric ester may still be of high molecular weight, the presence of minor amounts of impurities can significantly lower polymer melting point, a relatively high melting point, i.e. about 258.degree. C, being an important characteristic for high temperature processing.
In direct esterification, unforeseen problems can develop which are not present in the ester interchange process during the polymerization stages because of the high intrinsic viscosities generated which may be above 1 to 1.5. Paramount among such problems is the thermal sensitivity of the polycondensed product. As vacuum is increased and temperature raised during the final stages of polymerization, polymer degradation can eliminate the advantages gained through employment of the direct esterification method. Often, temperature is increased to within a few degrees of the degradation temperature ranges of the polymer. Hence, maximum molecular weight development requires extremely accurate control of the temperature of the molten mass under existing process conditions. At this point in the process, another factor not found with the ester interchange process cooperates with temperature to accentuate the thermal problem. This factor is end product viscosity. With intrinsic viscosities as aforementioned, melt viscosities as measured at polymerization temperature may range as high as 10,000 to 100,000 poises and are rarely below 2,000 poises. The power required within the reaction vessel to agitate this extremely viscous polymeric mass can in itself offset the delicate balance between heat input and degradation. Thus, direct esterification requires precise control of process parameters and reaction equipment design not contemplated nor required by prior processes. Further, high molecular weight must be developed without undue residence time. A long residence time at high temperatures again causes polymer degradation resulting in a darkly discolored and non-uniform product. However, it would be expected that direct esterification; of necessity, would involve long residence times, and, that this would be another factor which must be overcome before commercially acceptable product suitable for the desired end uses can be prepared.
It is an object of the present invention to provide a continuous, integrated process to produce polyesters of high intrinsic viscosity and of suitable properties for conversion into shaped articles, particularly fibers and films. It is another object of this invention to provide a continuous process to produce high molecular weight polyesters wherein monomer for polycondensation is prepared by direct esterification of a dicarboxylic acid with a glycol.
Still another object of the present invention is to provide a reproducible process to produce polyethylene terephthalate of fiber and film-forming quality having intrinsic viscosities heretofore unobtainable.
A further object of this invention is to provide a method for producing polyesters of high intrinsic viscosity involving direct esterification and polycondensation stages wherein side reactions are minimized and means are provided to enable the attainment of high molecular weights within reasonable residence times and under conditions preventing product degradation.
Other objects of the invention will appear obvious from the detailed description of the invention hereinafter.